USB mains charger for Sony Reader

I have a Sony reader PRS-T1. Recommended voltage of 3.7 volt DC. I use a standard computer USB output but wish to use a universal mains to USB charger that appears to have a 5 volt output. Other discussions give information mainly for mobile phones. Can anybody advise me please on using USB with the Sony Reader. Thanks

All PC USB ports output 5v. In order to charge a battery a voltage higher than that of the battery is necessary hence the difference. If any device is charged via a PCs USB port then a mains to USB charger is OK to use.